As materials for transparent conductive coating compositions, there are known tin oxide particles, antimony-containing tin oxide particles, tin-containing indium oxide particles, zinc oxide particles substituted by aluminum, etc. Among these materials, tin-containing indium oxide particles are used for coatings applied to the screens of cathode-ray tubes (CRT) and liquid crystal displays (LCD) which are required to have antistatic properties and electromagnetic wave-shielding properties, because the tin-containing indium oxide particles have high translucency to visible light and high electric conductivity. Further, sheets having the tin-containing indium oxide particles dispersed and applied thereon are used for not only displays but also a wide variety of other applications such as touch panels, because of their translucency and conductivity.
However, the properties of coating films comprising the tin-containing indium oxide particles are inferior to tin-containing indium oxide films formed by a vapor deposition process or a sputtering process, and thus, the application of such coating films has been limited, because the advantage that the coating films can be formed by a relatively simple and inexpensive method, i.e., coating, has not been fully utilized. The tin-containing indium oxide particles have another problem in the higher cost of raw materials since indium as a main raw material is expensive.
On the other hand, zinc oxide particles, titanium oxide particles, cerium oxide particles, iron oxide particles, etc. are known as materials for UV-shielding or highly refractive coating compositions. Among these materials, zinc oxide particles show superior shielding properties to UV rays within the region of UV-A, and particularly show high transparency to visible light. Therefore, the zinc oxide particles are used as UV-shielding cosmetic materials and are further used as highly refractive materials because of their high refractive index (2.1).
When these transparent particles such as tin-containing indium oxide particles and zinc oxide particles are dispersed in a binder for application, it is needed that the particle size thereof should be generally at most a half of the wavelength of visible light in order to obtain high transparency to visible light. Accordingly, in order to be transparent to, for example, visible light, these particles should have a particle size of as small as 200 nm or less.
One of typical methods for manufacturing such fine particles is disclosed in JP-A-62-7627. According to this method, an aqueous alkaline solution such as an aqueous ammonia, an aqueous ammonium carbonate solution or the like is added to an aqueous solution of a mixture of indium chloride and tin chloride to form a co-precipitated hydroxide; the co-precipitated hydroxide is then treated by heating to form tin-containing indium oxide; the tin-containing indium oxide is mechanically ground to obtain fine particles thereof. In the method of JP-A-62-7627, tin-containing indium oxide particles having an average particle size of 0.1 μm are obtained by the heat treatment and the mechanical grinding.
According to JP-A-5-201731, the co-precipitated hydroxide of indium and tin is obtained in the same manner as in JP-A-62-7627, and then is baked and ground to obtain tin-containing indium oxide particles, while it is important in this method that the contents of sodium and potassium should not be larger than a specified amount in order that the resultant particles can have high conductivity. In the method of JP-A-5-201731, tin-containing indium oxide particles having a particle size of 0.01 to 0.03 μm are obtained after the grinding.
On the other hand, it is known that zinc oxide fine particles themselves have strong coagulating power and thus are hard to disperse. In order to improve the dispersibility of zinc oxide fine particles, a very small amount of an oxide or an hydroxide of silicon or aluminum is contained into the respective zinc oxide particles (see JP-A-201382). Thereby, zinc oxide particles having a particle size of not larger than 0.03 μm and having sufficient dispersibility are obtained.
For example, these transparent conductive particles and UV-shielding and highly refractive particles are used for anti-reflection films having excellent antistatic effect. While a conventional anti-reflection film of this type is obtained by laminating a plurality of layers having individual functions, recently, an anti-reflection film having a plurality of functions in a single layer structure is desired in association with the development of a variety of thin-shaped appliances. For example, JP-A-2002-16757 discloses such a single-layer film. In this publication, a highly refractive and conductive material is obtained by dispersing, in a binder, conductive fine particles which comprise indium oxide and tin oxide as main components and highly refractive particles which comprise titanium oxide and zinc oxide. In this method, it is necessary that the sizes of the particles should not be larger than 0.2 μm, and the thickness of a coating film should not be larger than 20 μm in order to maintain the transparency of the film and the dispersibility of the particles. In Example 1 of this publication, a coating composition is prepared by mixing and dispersing tin-containing indium oxide particles and cerium oxide particles, and a coating film having a refractive index of 1.68 and a surface resistance of 2.5×109 Ω/□ is formed by applying this coating composition.
In the above mixture dispersion system of conductive particles and non-conductive particles, the non-conductive particles are held between the conductive particles by mixing and dispersing the non-conductive particles and the conductive particles, so that the contacts between each of the conductive particles are decreased. As a result, the electric conductivity of the resultant coating film tends to lower. This is one of the essential features of the mixture dispersion systems of this type. As for the antistatic effect alone, a coating film having a surface resistivity of about 109 Ω/□ has sufficient conductivity, in other words, an antistatic function, and is expected to have further functions such as an electromagnetic wave-shielding property, or conductivity so high as to be applicable to a touch panel or the like. However, to obtain such excellent conductivity, it is needed to increase the content of conductive particles of tin-containing indium oxide or the like as much as possible. However, disadvantageously, an increase of the content of the conductive particles leads to a decrease of the content of non-conductive particles such as zinc oxide or the like. As a result, the UV-shielding effect, i.e., one of the features of the zinc oxide is hardly exhibited. In this way, there is a relationship of trade-off between the conductivity and the UV-shielding function.
In these years, there is an increased demand for antistatic coating compositions comprising white conductive particles, for use in white garment and the interior decoration of clean rooms required to have cleanliness, or bright and vivid color coating compositions or plastic products. Examples of the white conductive particles for use in these applications are white inorganic pigment particles each of which comprises a core of potashmica coated with tin-containing indium oxide (JP-A-60-253112), white conductive particles each of which comprises a white inorganic pigment particle of zinc oxide, titanium oxide or the like, coated with tin dioxide and further coated with tin-containing indium oxide (JP-A-06-338213), etc. Some of these white conductive particles are manufactured by coating the outer surfaces of UV-shielding white pigment particles of titanium oxide or zinc oxide as cores with conductive tin oxide, tin-containing indium oxide or the like. Theses particles are manufactured for the purpose of obtaining white colors but are not used as UV-shielding agents. Further, because of the influence of the white inorganic pigment particles as base substances, it is difficult to sufficiently lower the volume resistivity of the particles to a level necessary for use in conductive coating composition or the like.